Such fluid working machines are generally used, when fluids are to be pumped or fluids are used to drive the fluid working machine in a motoring mode. The word “fluid” can relate to both gases and liquids. Of course, fluid can even relate to a mixture of gas and liquid and furthermore to a supercritical fluid, where no distinction between gas and liquid can be made anymore.
Very often, such fluid working machines are used, if the pressure level of a fluid has to be increased. For example, such a fluid working machine could be an air compressor or a hydraulic pump.
Generally, fluid working machines comprise one or more working chambers of a cyclically changing volume. Usually for each cyclically changing volume, there is provided a fluid inlet valve and a fluid outlet valve.
Traditionally, the fluid inlet valves and the fluid outlet valves are passive valves. When the volume of a certain working chamber increases, its fluid inlet valve opens, while its fluid outlet valve closes, due to the pressure differences, caused by the volume increase of the working chamber. During the phase, in which the volume of the working chamber decreases again, the fluid inlet valve closes, while the fluid outlet valve opens due to the changed pressure differences.
A relatively new and promising approach for improving fluid working machines is the so-called “synthetically commutated hydraulic pumps”, also known as “digital displacement pumps”. These pumps are a subset of variable displacement pumps. Such synthetically commutated hydraulic pumps are known, for example, from EP 0 494 236 B1 or WO 91/05163 A1. In such pumps, the passive inlet valves are replaced by electrically actuated inlet valves. Optionally the passive fluid outlet valves are also replaced by electrically actuated outlet valves. By appropriately controlling the valves, a full-stroke pumping mode, an empty cycle mode (idle mode) and a part stroke pumping mode can be achieved. Furthermore, if both inlet and outlet valves are electrically actuated, the pump can be used as a hydraulic motor as well. If the pump is run as a hydraulic motor, full stroke motoring and part-stroke motoring is possible, as well.
A major advantage of such synthetically commutated hydraulic pumps is their higher efficiency, as compared to traditional hydraulic pumps. Furthermore, because the valves are electrically actuated, the output characteristics of a synthetically commutated hydraulic pump can be changed very quickly.
For adapting the fluid flow output of a synthetically commutated hydraulic pump according to a given demand, several approaches are known in the state of the art.
It is possible to switch the synthetically commutated hydraulic pump to a full pumping mode for a certain time for example. When the synthetically commutated pump is operated in a pumping mode, a high pressure fluid reservoir is filled with fluid. Once a certain pressure level is reached, the synthetically commutated hydraulic pump is switched to an idle mode and the fluid flow demand is supplied by the high pressure fluid reservoir. As soon as the pressure of the high pressure fluid reservoir reaches a certain lower threshold level, the synthetically commutated hydraulic pump is switched on again.
This approach, however, necessitates a relatively large high pressure fluid reservoir. Such a high pressure fluid reservoir is expensive, occupies a large volume and is quite heavy. Furthermore, a certain variation in the output pressure will occur.
So far, the most advanced proposal for adapting the output fluid flow of a synthetically commutated hydraulic pump according to a given demand is described in EP 1 537 333 B1. Here, it is proposed to use a combination of an idle mode, a part-stroke pumping mode and a full-stroke pumping mode. In the idle mode, no fluid is pumped by the respective working chambers. In the full-stroke mode, all of the usable volume of the working chamber is used for pumping during the respective cycle. In the part stroke mode, only a part of the usable volume is used for pumping during the respective cycle. The different modes are distributed among several chambers and/or several successive cycles in a way, that the time averaged effective flow rate of fluid through the machine satisfies a given demand.
In controlling methods, which have been employed so far, a fluid flow demand, usually expressed as the displacement demand, is used as the (main) input parameter. The displacement demand is expressed as a certain percentage of the maximum displacement of the fluid working machine. The displacement demand is given by e.g. the position of a command (e.g. joy stick, pedal, throttle or the like), operated by an operator. In the controller, the displacement demand, which is expressed as a certain percentage of the maximum displacement of the fluid working machine is considered by using the so-called “accumulator” variable. The accumulator sums up the demand in a variable, used in an electronic controller unit, controlling the operation of the fluid working machine. As soon as a certain threshold level of the accumulator has been reached, a pumping cycle of the next following working chamber is initiated and the accumulator is decreased by an amount, corresponding to the volume to be pumped.
In the very first synthetically commutated hydraulic pumps, only idle strokes and full-stroke pumping cycles were used. Here, the accumulator integrated the fractional demand. As soon as the accumulator exceeded 100%, a full stroke pumping cycle was initiated and the accumulator would be decreased by 100%, accordingly.
In EP 1 537 333 B1 an additional part stroke mode of a certain, previously defined displacement fraction was suggested. Here, depending on the demand and the value of the accumulator, a part stroke or a full stroke pumping cycle would be initiated and the accumulator would be decreased by an appropriate value.
However, in practical applications, the control algorithms known in the state of the art have severe drawbacks, especially under certain working conditions.
One major drawback is pulsations, positive and negative pressure spikes occurring under certain working conditions. If, for example, the demand is very low, it takes a very long time for the accumulator to rise to a value beyond the threshold, before a stroke is finally initiated. The resulting pressure variations can be noticed during the movement of a hydraulic consumer (e.g. a hydraulic piston or a hydraulic motor). Also, a start-stop movement (a “sticking” behaviour) can be noticed. The pressure pulsations can even lead to the destruction of certain parts of the hydraulic system.